1. Field of the Invention
This invention relates generally to direct methanol fuel cells, methods for producing electricity using those fuel cells, and fuel compositions for direct methanol fuel cells (DMFCs) including additives to those fuels.
2. Background Information
Fuel cells are devices in which electrochemical reaction involving fuel molecules and oxygen are used to generate electricity. Methanol is an attractive fuel choice for some applications due to the its high specific energy and the relatively fast electrokinetics with which it can be oxidized to generate electricity.
In direct oxidation fuel cells, a carbonaceous liquid fuel (typically methanol or an aqueous methanol solution) is introduced to the anode face of a membrane electrode assembly (MEA). Those skilled in the art will recognize that other compounds and mixtures may be added to the methanol or methanol/water fuel, and may be used as fuels, such as ethanol and other alcohols and alcohol precursors, and aqueous solutions thereof.
Examples of DFMCs include the Vapor Feed Fuel Cell System with Controllable Fuel Delivery, disclosed in commonly assigned Ser. No. 10/413,986, filed Apr. 15, 2003, and Direct Oxidation Fuel Cell Operating with Direct Feed of Concentrated Fuel under Passive Water Management, Ser. No. 10/413,983 filed Apr. 15, 2003 the disclosures of which are incorporated herein by reference.
Methods for refueling fuel cells and fuel cell powered devices are disclosed in commonly assigned application Methods of Providing Refueling for Fuel Cell Powered Devices, Ser. No. 10/607,699 filed Apr. 15, 2003 the disclosure of which is incorporated herein by reference.
Methanol, CH3OH (C.A.S. No. 67-56-1) is a clear, colorless, flammable liquid or vapor that is toxic and can be harmful or fatal if inhaled, adsorbed through the skin or swallowed. Methanol has a flash point of 11° C., and autoignites at 464° C. Concentrations of about 6 to about 36 percent by volume burn in air if ignited. Methanol is a Class 3 Flammable liquid, U.N. number 1230, and transportation of methanol is regulated by the U.S. Department of Transportation and the International Civil Aviation Organization. Presently, transportation of methanol is prohibited in the passenger compartments and checked baggage of passengers travelling by air. Current DOT regulations provide 24% aqueous solutions of methanol are not hazardous, but ICAO regulations do not make any such provision.
As use of DMFCs increases in small electronic devices, large numbers of fuel cells, each containing methanol, will be exposed to the public in a wide variety of environments and circumstances.
A fuel cartridge or fuel tank placed in a direct methanol fuel cell anode compartment and containing a highly concentrated methanol solution or neat methanol will absorb or pick up water produced from the fuel cell electrode reaction. A membrane that separates the highly concentrated methanol solution or neat methanol within the tank or cartridge from the cell anode is called methanol delivery membrane or film. Water absorption across the methanol delivery membrane is driven by the water concentration or activity difference between the cell electrode and the highly concentrated methanol solution or neat methanol. The consequence of water uptake in the fuel cartridge or fuel tank is dilution of the methanol by water inside the fuel tank or cartridge. This decrease in the methanol concentration in turn causes a decrease in methanol transport rate through the membrane, and thus the methanol feed rate to the cell anode. Consequently, the fuel cell power output decreases during cartridge life. Moreover, the power demand on a fuel cell is not constant, and in many applications a power surge is demanded when the circuit is closed to allow the device to start up. Although the water uptake can be minimized to a level by using a membrane that is highly selective to the transport of methanol over water, completely eliminating water uptake may be difficult to achieve using materials that are currently commercially available. As such, we have now found it may be advantageous or desirable to add certain compounds to the fuel in order to improve the effectiveness of the fuel cell system under given conditions.
Direct oxidation fuel cell systems for portable electronic devices should be as small as possible at the power output required. The power output is governed by the rate of the reactions that occur at the anode and the cathode of the fuel cell. More specifically, the anode process in direct methanol fuel cells based on acidic electrolytes, including polyperflourosulfonic acid and similar polymer electrolytes, involves a reaction of one molecule of methanol with one molecule of water. In this process, the oxygen atom in the water molecule is electrochemically activated to complete the oxidation of methanol to a final CO2 product in a six-electron process, according to the following chemical equationCH3OH+H2O=CO2+6H++6e−  (1)
Since water is a reactant in this anodic process at a molecular ratio of 1:1 (water:methanol), the supply of water, together with methanol, to the anode at an appropriate weight (or volume) ratio is critical for sustaining this process in the cell. In fact, it has been known that the water:methanol molecular ratio in the anode of the DMFC has to significantly exceed the stoichiometric, 1:1 ratio shown by process (1). This excess is required to guarantee complete, 6 electron anodic oxidation to CO2, rather than partial oxidation to either formic acid, or formaldehyde, 4e− and 2e− processes, respectively, described by equations (2) and (3) below:CH3OH+H2O=HCOOH+4H++4e−  (2)CH3OH=H2CO+2H++2e−  (3)
Equations (2) and (3) describe partial processes that are not desirable and which might occur if anode water content is not sufficient during a steady state operation of the cell. Particularly, as is indicated in process (3), involving the partial oxidation of methanol, water is not required for this anode process and thus, this process may dominate when the water level in the anode drops below a certain point. The consequence of process (3) domination, is an effective drop in methanol energy content by 66% compared with consumption of methanol by process (1), which would result in a lower cell electric energy output. In addition, it might lead to the generation of a hazardous anode product (formaldehyde).
U.S. Pat. No. 6,554,877 discloses a liquid fuel composition for electrochemical fuel cells including dilute aqueous solutions of methanol with an auxiliary fuel that is a hydrogen containing inorganic compound.
U.S. Pat. No. 5,904,740 discloses a fuel for fuel cells comprising methanol and formic acid that is substantially free of mineral acids.
U.S. Pat. No. 5,599,638 discloses aqueous methanol solutions for fuel cells optionally including additives.
It is thus an object of the present invention to provide a concentrated (i.e. a fuel that contains a fuel with a methanol:water ratio of at least 1: 1) fuel and fuel additives for DMFCs that overcome the above difficulties to provide extended life and increase energy density and power output for fuel cell systems.
Another object of the present invention is to provide a fuel that includes organic compounds that undergo rapid reaction with water to yield hydrocarbons including methanol suitable for fuel in the cell. In this aspect, the fuel mixture will allow improved consistency of fuel delivery to the anode of the fuel cell by reacting with water that passes from the anode aspect of the fuel cell into the fuel mixture. By doing so, a more consistent mixture of fuel will be delivered to the fuel cell system, resulting in improved uniformity of power output from the cell as fuel is consumed. Moreover, because there is preferably little or no water in the fuel mixture, these fuel mixtures are more stable than in other fuels for direct oxidation fuel cell systems.
In another object of this invention, the improved uniformity of fuel delivery will reduce or eliminate the period of low power output near the end of a fuel supply's useful life.
Still another advantage of the present invention is the reduction and elimination of the potential release of methanol fuel from cartridges that are disposed of improperly when a user perceives that the cartridge is exhausted, because the concentration of methanol is too low to provide a feed rate to the cell that provides acceptable power even though the cartridge contains too high a concentration of methanol to be discarded safely.
Another object of the present invention is to provide improved safety of DFMCs by including an indicator in the fuel that will signal a person that there has been a release of methanol fuel from the fuel cell cartridge.
Still another object of the present invention is to provide improved fuel cell power pack energy content by enabling improved fuel delivery from the fuel cartridge to the fuel cell system.